[BUG] CLI test fixtures leaking processes on Windows

Description

CLI subprocess tests leave OpenCode processes running on Windows after the suite finishes. I currently have 35 of them alive, oldest 46 hours, ~32 GB of private commit between them, still holding loopback ports and file locks on checkouts.

The cause is in packages/opencode/test/lib/cli-process.ts. It spawns children with Bun.spawn(["bun", ...]). When the suite is launched through bun x (bunx), PATH resolves "bun" to the small node_modules\.bin\bun.exe shim, which starts the real bun as a separate process and waits on it. The fixture only ever holds the shim. So kill() + await exited completes as soon as the shim dies, while the real OpenCode process keeps running with no parent.

Two helpers leak: serve() and startRun(). acp() happens to be safe because stdin EOF exits the child first, and the generic spawn() is safe because it goes through CrossSpawnSpawner, which tree-kills on Windows.

The two tests meant to catch this can't:

• test/cli/serve/serve-process.test.ts asserts typeof code === "number" || code === null, which is true for every possible value.
• test/cli/run/run-process.test.ts has a test named "SIGINT interrupts an active non-interactive run without leaking the process" that leaks one every run.

I have a fix ready and will open a PR against this issue.

Plugins

custom

OpenCode version

1.17.11

Steps to reproduce

1. On Windows, from packages/opencode, run the suite through bunx so the shim is on PATH:
   bun x bun@1.3.14 test test/cli/serve/serve-process.test.ts -t "kills the subprocess on scope close"
2. The test passes.
3. List bun processes:
   Get-CimInstance Win32_Process -Filter "Name='bun.exe'" | Select-Object ProcessId, ParentProcessId, CommandLine
4. There is a leftover ... src\index.ts serve --port 0 process whose parent no longer exists. It still answers HTTP on its port.

Screenshot and/or share link

No response

Operating System

Windows 10

Terminal

Windows Terminal (PowerShell)